Endosseous root form implants—hereinafter referred to as “implants”—are well known devices, which are adapted to support and receive dental prosthetic pieces. Root form implants are generally cylindrical, with a surface area designed to promote good attachment to the jawbone. Most implants are made of titanium or of a titanium alloy, due to the biocompatibility and high rate of osseointegration, i.e. the physiological process of fusing with a living bone, of such a material.
Non-expandable implants have a continuous, cylindrically shaped body, which is threadedly fitted or press-fitted in a bore drilled within a jawbone. The prosthetic piece may not be received in the implant for a period of approximately five months, until bone tissue of the jawbone grows and eventually fuses with the implant, causing discomfiture to a patient and necessitating several visits to an oral surgeon. During this period, bone resorption is noticeable, due to the lack of pressure normally applied by the extracted tooth onto the bone tissue. At times, the bore is improperly drilled, and is much larger than the fixed diameter of the implant. As a result, the implant will not be properly secured to the jawbone. Micro-movement of the implant relative to the wall of the bore is another cause of implant looseness.
The use of an expandable implant, whereby the outer diameter of the implant relative to the wall of the bore is adjustable, obviates the aforementioned disadvantages of non-expandable implants. By providing a controlled amount of lateral expansion, the outer surface of the implant is in pressured, frictional engagement with the jawbone. The degree of frictional engagement with the jawbone is increased by the shape of an expanded implant, which is similar to that of the root of a tooth. Therefore the implant may receive a corresponding prosthetic piece and provide sufficient functional loading for mastication immediately after expansion of the implant, due to the securing of the implant to the jawbone. When a prosthetic piece is attached to the implant immediately following a tooth extraction, bone tissue growth, and consequently osseointegration, is induced by blood flow in the jawbone, the rate of which is substantially equal to the blood flow rate prior to tooth extraction. A substantially unchanging blood flow rate in the jawbone increases the rate of implant osseointegration with the jawbone. Exemplary expandable implants are disclosed in U.S. Pat. Nos. 3,708,883, 5,004,421, 5,470,230, 5,489,210, 5,931,674, 5,951,288 and 6,506,051.
The lateral expansion of the prior art implants is effected by a tubular envelope having deformable elements at the apical end thereof and by an expander member, generally of a frusto-conical shape, in engagement with said deformable elements. As the expander member is displaced along the longitudinal axis of the tubular member, the expander member forces the deformable elements to expand outwardly against the interior sidewall of the jawbone hole.
The securing means, by which the expander member of the prior art expandable implants is secured to the tubular member, is formed longitudinally above the expander. That is to say, the length of an implant is increased due to the arrangement of the securing means relative to the expander member. A short implant is of particular importance for those patients having thin-walled or deformed jawbones, e.g. due to osteoporosis or bone resorption, since a bore drilled in the jawbone in order to receive a normally sized implant of the prior art is liable to injure the mandibular nerve or penetrate the sinus lining. Approximately 20-30% of those patients who require dental implants have thin-walled or deformed jawbones, and therefore these patients are precluded from receiving prior art dental implants.
Prior art dental implants generally have a tubular barrel, which is screwed into a bore formed in a jawbone, having a plurality of radial slits formed at the end which enters furthest into the bore and threads formed on its interior surface. An expander screw has a frusto-conically shaped end surface and a cylindrical outer surface positioned between coronal threads and the end surface. The expander screw is shaped to engage and mate with the threads formed on the interior surface of the barrel, such that advancement of the expander screw along the barrel toward the insertion end causes an end surface of the expander screw to collide with the interior surface of the barrel and to expand the insertion end outward into the surrounding bone. The slits reduce the available barrel surface area that engages the sidewall of the bore, and also weaken the structure of the implant. Due to the reduced structural strength of the implant, each leg of the barrel formed between adjacent radial slits may twist and sever upon removal of the implant, if implant removal is deemed necessary.
Furthermore, the exterior surface of the barrel, which is not immobilized, is liable to be loosened from the sidewall of the jawbone bore over the course of time. Since the expander screw mates with the threading formed on the interior surface of the barrel, reverse rotation of the expander screw, i.e. in a rotational direction opposite to that which causes the barrel to press into the sidewall of the jawbone, due to inter-jaw dynamic compressive forces and fluctuating stresses, results in a loosening of the implant.